High strength joined porous block structures and method of making same

ABSTRACT

A HIGH STRENGTH STRUCTURE SUITABLE FOR USE AS AN ELECTRODE IS MADE FROM A PLURALITY OF POROUS BLOCKS JOINED TOGETHER BY A SEAM MIXTURE WHICH INCLUDES A CARBONIZABLE BINDER MATERIAL. BEFORE THE BLOCKS ARE JOINED TOGETHER, SURFACES OF THE BLOCKS ARE COATED WITH A MASTIC COMPRISING A MIXTURE OF A CARBONIZABLE BINDER MAY FINELY DIVIDED HEAT RESISTANT PARTICLES, THE PARTICLES FORMING 30 TO 75 PERCENT BY WEIGHT OF THE MASTIC. THE ASSEMBLY, I.E., THE COATED BLOCKS AND SEAM MIXTURE IS THEN BAKED FOR A LENGTH OF TIME AND AT A TEMPERATURE SUFFICIENT TO CARBONIZE THE BINDERS.

April 27,,- 1971 M. B. DELL 3,576,700

HIGH STRENGTH JOINED POROUS BLOCK AND METHOD OF MAKING SAME Filed Nov. 5, 1969 l g I 9 3% g .4 2 2 p g a! 3 I FIG.|.

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a: v i l 1 l 1 :0 so 40 so so ANTHRACITE FINES, BY WEIGHT F|G.2. INVENTOR M. Benjamin Dell ATTORNEY United States Patent HIGH STRENGTH JOINED POROUS BLOCK STRUCTURES AND METHOD OF MAKING SAME Manuel Benjamin Dell, Pittsburgh, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa.

Filed Nov. 3, 1969, Ser. No. 873,243 Int. Cl. B32t 3/14 US. Cl. 161-38 9 Claims ABSTRACT OF THE DISCLOSURE A high strength structure suitable for use as an electrode is made from a plurality of porous blocks joined together by a seam mixture which includes a carbonizable binder material. Before the blocks are joined together, surfaces of the blocks are coated with a mastic comprising a mixture of a carbonizable binder and finely divided heat resistant particles, the particles forming 30 to 75 percent by weight of the mastic. The assembly, i.e., the coated blocks and seam mixture is then baked for a length of time and at a temperature sufiicient to carbonize the binders.

BACKGROUND OF THE INVENTION The present invention relates in general to carbonaceous structures, and particularly to a high strength joint between blocks of porous material, such as prebaked carbon blocks forming an electrode structure. The invention is especially applicable to wide joints, such as is necessitated by use of tamping tools to achieve high green density of the joint material.

As is well known, in lining the interior of electrolytic cells for the production of aluminum by the Hall-Heroult process via electrolysis of A1 0 dissolved in a molten bath of cryolite, prebaked carbon blocks are placed in the bottom of the cell, and then joined together to form a unitary structure which becomes the cathode liner of the cell. The carbon blocks are joined together by what is generally known as a seam mix, the mix usually comprising a cokable binder in which is mixed a carbonaceous aggregate. Whenthe blocks are placed in the bottom of the cell, they are positioned to leave a space between adjacent blocks, the seam mix is then poured and tamped or otherwise compacted in the spaces, and the assembly of blocks and joints is baked to carbonize the binders and thus join the individual blocks together to form the unitary cathode structure. In this manner, the carbon cathode structure serves as the bottom and side liner of a vessel designed to contain the molten salt bath (of the cell) and the molten aluminum produced therein. For this reason, it is important that no weakening, resulting in cracks and leaks, occur in the liner.

A weak point in such liners has been the seam joints between the carbon blocks. With the operation of the cell, the joints have opened in relatively short periods of time requiring frequent repairs and replacement. This has represented a significant maintenance and operating burden in the production of aluminum.

Investigation disclosed that the weakest part of the seam was at the interface between the seam mix and the carbon block. It was further found that the weakness was caused by a local deficiency of the binder due to its migration and absorption into the pores of the block from the seam mix, and from a pitch precoat when used.

BRIEF SUMMARY OF INVENTION The present invention provides a simple and inexpensive method and means by which the problem of binder migration into the block is reduced to a negligible amount, and

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the joint between the blocks thereby substantially strengthened to provide a more durable, failure-resistant liner. This is accomplished by first coating the carbon blocks with a mastic material (comprising a binder mixed with an aggregate of heat resistant, finely divided refractory particles in proportions hereinafter described) before the seam mix is compacted in place between the blocks. After the blocks are coated, disposed in place, and the seam mix compacted between the blocks, the assembly is baked for a period of time and at a temperature level suflicient to coke or carbonize the binder materials in the seam and mastic coating.

The coating of mastic material on the block greatly reduce absorption of the binder component into the block by virtue of the refractory, heat resistant particles contained therein. The size and amount of particles mixed with the binder is such that the viscosity of the mastic is increased to the extent that it will not flow in substantial amounts into the pores of the carbon blocks even at elevated temperatures. The mastic coating with the aggregate thus acts as a barrier to the flow of the binder from the seam mix. In this manner, the binder remains in the seam mix and at the interface between the seam and the block to help provide a strong, enduring bond with continued use and operation of the cell.

DESCRIPTION OF THE DRAWING The invention, with its objectives and advantages, will be more apparent from the following detailed description taken in connection with the accompanying drawing in which:

FIG. 1 is a vertical section of seams joining porous blocks in accordance with the principles of the present invention; and

FIG. 2 is a graph showing the flexural strength achieved with varying amounts of heat resistant aggregate in a binder material used to precoat the porous blocks of FIG. 1 before they are joined together.

PREFERRED 'EMB ODIMENT In FIG. 1 there are shown three blocks 1 made of a porous material joined together with a seam material or mixture 2 to form a composite structure generally designated 3. The blocks 1, for example, may be prebaked carbon blocks joined together to form a cathode liner for an electrolytic cell (not shown) used in the production of aluminum though the invention is not limited thereto. The seam material 2 may be any of the well-known seam mixes employed to join carbon blocks together. A typical seam mix comprises a mixture of a binder such as coke-oven pitch and an aggregate such as calcined anthracite.

In accordance with the invention, before the blocks 1 are brought into place and their surfaces juxtaposed to form confined spaces therebetween for receiving the seam mix 2, the juxtaposed surfaces are provided with a coitt ing or precoat 4 of mastic material which serves as a. barrier to the flow or absorption of the binder into the pores of the blocks. This is accomplished by using as the mastic coating, a mixture of a carbonizable binder material, such as a coke-oven pitch, and an aggregate of refractory particles, such as finely divided (preferably mesh screen) calcined anthracite fines, though other binders and aggregates may be used as explained hereinafter.

After the blocks 1 are precoated wtih the mastic 4, the blocks are placed together in the manner indicated in FIG. 1. With the' seam mix 2 suitably compacted in place between the blocks, the assembly 3 is baked to coke the binders in the mastic coat and seam to form a strong, durable bond between each pair of blocks.

The strong bond is formed by virtue of the binder material in the seam 2 and in the mastic precoat 4 remaining in the seam and at the interface between the seam and block 1 as explained above, i.e., the seam mix experiences little flow into the block. It is believed the reason for this is that the aggregate increases the viscosity of the mastic to the point that the binder cannot flow substantially into the block even at the elevated temperatures encountered during the baking process. Also, it is possible that with any absorption of the binder by the blocks, the binder transports the fine particles contained in the mastic into the pores to immediately fill the same and thus substantially limit any further flow of the binder. In any case, the binder remains in the seam with the result being a seam joint 4 having a flexural strength many times that of prior art structures.

The conventional practice has been to use no precoat or a precoat of a pure binder such as pure pitch, i.e., a pitch with no (zero percent) aggregate. Samples of joints made with a precoat of pitch only, were tested to determine their flexural strength which strength ranged from to 60 p.s.i. as explained further hereinafter. One such joint fell apart during the baking process. By adding an aggregate of refractory particles to the precoat, joint strength approached 375 p.s.i. as indicated by the graph of FIG. 2. As indicated, a preferred composition for the precoat is 45 to 70 percent by weight of aggregate the remainder of the precoat being a complement of binder material, the binder and aggregate comprising the total mastic composition. However, as is well known in the art, a suitable portion of graphite can be added to improve electrical conductivity in the seam 2 and precoat 4.

The preferred binder for the mastic coating or precoat is coke-oven pitch having about a 75 degree centigrade softening point although pitches with other softening points may be used as well as other suitable binders. Such other binders include furan resins as well as other commercially available cements and bonding materials.

The preferred heat resistant aggregate is calcined anthracite though other refractory particles may be used, for example, coke, graphite or alumina. In selecting the aggregate for blocks in aluminum smelting cells, however, substances that might contaminate or otherwise adversely affect the smelting process and the quality of the metal produced thereby should be avoided.

A preferred procedure of joining the blocks 1 to form a strong, durable bond is as follows: The blocks are first warmed to a temperature of about 80-130 degrees centigrade, the sides of the blocks preferably having smooth, clean surfaces. The sides of the warm blocks are next coated with approximately 0.3 pound of mastic per square foot of block surface area, the mastic being preheated to a temperature above the softening point of its binder component. The blocks are then allowed to cool and thereafter placed in a cell to form a liner therein. With the temperature of the blocks about 70 degrees centigrade, a seam mix 2 is added between the blocks, the mix comprising a sized anthracite aggregate (the bulk of which passes a 4 mesh Tyler screen) and approximately 14 percent by weight of a coke-oven pitch binder having a 40 degree centigrade softening point. The seam mix is added at a temperature of at least 130 degrees centigr-ade. Since the mix cools rapidly after being placed in contact with the relatively cool blocks, it should be added in small increments and tamped (compacted) immediately. To check the quality of the tamping operation, occasional measurement of GAD (green apparent density) may be taken at various levels in the seam 2. The final layer is tamped with a flat tool. The liner is then baked in the cell in a substantially inert atmosphere for several hours and at a temperature of 900 to 1000 (and above) degrees centigrade which is sufficient to carbonize the binder in the seam mix 2 and in the mastic precoat 4.

'For a spray application of the mastic precoat 4 to the 4 blocks 1, the mastic can be diluted with a few percent high flash coal tar solvent naphtha.

As indicated above, flexural strength tests were conducted on the joint samples made in accordance with the invention as well as those made with a precoat of binder material having a zero percent aggregate. The results of these tests are indicated by the graph of FIG. 2 which shows flexural strength of joints plotted against the percentage of aggregate in the total mastic. Each sample tested is indicated by an X on the graph, each X marking the strength at which the sample cracked or broke. The samples were tested by loading the center portions thereof in accordance generally with ASTM C133 test specifications.

As shown, the joints between those blocks having only a precoat of binder failed at very low flexural strengths, the highest value being p.s.i. whereas the joints between blocks having precoats containing increasing percentages of aggregate had increased strengths by substantial amounts. Thus, three blocks with 60 percent aggregate precoat had strengths concentrated in the neighborhood of 365 p.s.i. which is six times that of the best result attained with a precoat having no aggregate. As evidenced by the graph, however, there was some degree of scatter in the results of the tests so that not all of the aggregate samples had strengths in the 365 p.s.i. range. As indicated, a preferred range would be 45 to percent aggregate, though the results of the tests show a trend towards percent and higher. In all cases, however, the samples with some percentage of aggregate were substantially stronger than those having no aggregate in the precoat mastic, the samples with the aggregate precoat tending to concentrate in higher strength ranges.

It will be noted that the range of the aggregate percentage of the mastic, as shown in FIG. 2, has an upper limit of 70 percent, and that the line indicating the trend in strength (p.s.i.) ends at the top of the graph at 375 p.s.i. In testing various mastic precoats, it was found that when the percentage of aggregate in the mastic went much above percent, the mastic become too viscous to be workable. For this reason the graph does not cover further increases in the percentage of aggregate or give further results regarding flexural strength.

From the foregoing description it should now be apparent that a high strength joint has been disclosed which is particularly useful in forming durable electrode structures and liners for electrolytic cells. This is accomplished by use of a mastic coating (containing aggregate particles in amounts in a preferred percentage range of the total weight of the mastic) applied to porous blocks before the blocks are joined together with a seam mix, and baked to carbonize binder components forming a portion of the mastic coating and the seam mix.

Though the invention has been described with a certain degree of particularity, changes may be made therein without departing from the spirit and scope thereof.

Having thus described my invention and certain embodiments thereof, I claim:

1. A process of joining bodies of which at least one is a porous block, said process comprising the steps of coating surfaces of said bodies with a mastic material comprising a mixture of a carbonizable binder and finely divided, heat resistant particles,

bringing said bodies into juxtaposition with juxtaposed surfaces of said bodies defining a confined space therebetween, filling the spaces between said bodies with a seam mix having a carbonizable binder component, and

baking the bodies for a length of time and at a temperature value sufficient to carbonize the binders in the mastic coating and seam mix.

2. The process of claim 1 in which the mastic coating contains 30 to 75 percent by weight of the heat resistant particles.

3. The process of claim 1 in which the binder material in the mastic coating is pitch, and the heat resistant particles are calcined anthracite fines.

4. The process of claim 1 in which the bulk of the heat resistant particles passes a 100 mesh screen.

5. A process of joining bodies of which at least one is a porous block made from a carbonaceous material, said block having at least one side coated with a mastic material comprising a mixture of a carbonizable binder material and finely divided, heat resistant particles, said process comprising the steps of bringing said bodies into juxtaposition with juxtaposed surfaces of said bodies, including said mastic coated side, defining a confined space therebetween, filling the spaces between said bodies with a seam mix having a carbonizable binder component, and

baking the bodies and seam mix for a length of time and at a temperature value suflicient to carbonize the binder in the mastic coating and seam mix.

6. A high strength joined porous block structure comprising a plurality of porous blocks of carbonaceous material joined by a seam mix including a carbonized binder material between each two adjacent blocks of said plurality of porous blocks, and

at least one of said blocks having a carbonized mastic coating on the surface thereof between the seam mix and said surface,

said coating comprising a mixture of a carbonized binder material and finely divided, heat resistant particles.

7. The structure described in claim 6 in which the binder material in the mastic coating is carbonized pitch, and the heat resistant particles are calcined anthracite fines.

8. The structure described in claim 6 in which the heat resistant particles comprises to percent by weight of the mastic coating before the binder therein is carbonized.

9. The structure described in claim 6 in which the coated blocks and seam mix form a cathode assembly for an electrolytic cell.

References Cited UNITED STATES PATENTS 3,429,759 2/1969 Kellar et al. l56155 PHILIP DIER, Primary Examiner US. Cl. X.R.

l6l159, 182; l5689, 155, 337; 26429, 204- 294 Inventor(s) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,576,700 Dated April 27, 1971 Patent: No.

Manuel Benjamin Dell It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the Title Change "STRUCTURES" to --STRUCTURE--.

Col. 2, line 14 Change "reduce" to --reduces--.

Col. 3, line 33 Add -4-- after "precoat".

These three patents should be listed as references:

2,728,109 12/1955 Bonnot 264-29 3,055,789 9/1962 Gemmi 161-182Xr 3,275,488 9/1966 Bailey et a1 156-89 Signed and sealed this 7th day of September 1971 (SEAL) Attest:

EDWARD. F ROBERT GOTTSCHALK Attestlng Offlcer Acting Commissioner of Pate 

